Ms. Winfrey, who controls the second-largest stake in the New York company, is one of the most influential U.S. celebrities. Over the years, Ms. Winfrey had chronicled her weight struggles.

Under the terms of an initial agreement, Ms. Winfrey joined Weight Watchers’ board—and weight-losing program—and agreed to let Weight Watchers use her name and image. Weight Watchers’ shares, down 8% this year, have surged since Ms. Winfrey’s investment was announced. On Friday, shares rose 6% to $24.27 on the news of Mr. Cohen’s investment.

Over all, Mr. Cohen controls 3.8 million shares, the bulk of it through his investment firm Point72 Asset Management LP, according to a regulatory filing.

On Thursday, Weight Watchers raised its projections for the year following better-than-projected third-quarter results. Still, the New York company reported a steep decline in profit along with significant declines in other key metrics, including active subscribers, which fell 13% to 2.6 million.

Weight Watchers said it now expects to make 64 cents to 74 cents a share for the year. In 2014, it made $2.03 a share.

The company, which started in the early 1960s when a group of friends met in Queens, N.Y., to discuss how to lose weight, has struggled to recruit and retain paying members.

It has been trying to revamp operations around fitness and technology, building on personal coaching and 24/7 expert chats to foster the kind of online relationships its members had traditionally developed in face-to-face meetings.